1) Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus that can be used in a laser beam printer (LBP), a digital copier, a plain paper facsimile (PPF), and the like.
2) Description of the Related Art
In recent electrographic image forming apparatus such as a laser printer, densification of the image formed is speedy. To realize densification of the formed image, it is necessary to realize a small beam spot diameter for optical scanning on an image forming surface of an image carrier such as a photoconductor. The demand for such an optical scanner is increasing.
(1) To achieve a small beam spot diameter, Japanese Patent Application Laid-Open No. 2001-324689 proposes a two-lens scanning lens, with a special toric surface in which the radius of curvature in a vertical scanning section changes asymmetrically, toward the periphery of the horizontal scanning direction, from the optical axis of the lens surface, and the whole surface of the scanning lens is formed of the special toric surface.
(2) Further, Japanese Patent Application Laid-Open No. 2001-21824 proposes an optical scanner to realize a stable and favorable optical spot by correcting the wavefront aberration, where at least one surface of a lens included in a scanning imaging optical system is a vertical non-arc plane such that the shape in the horizontal scanning section is an arc or non-arc, and the shape in the vertical scanning section is non-arc, and the vertical non-arc plane is formed such that an incident angle of the principal ray in deflected beams entering into the respective lens surfaces of the lenses in the scanning imaging optical system, with respect to the normal on the lens surface, is 25 degrees or less in the whole area of the lens effective area.
However, the scanning lens using the conventional special toric surface, as described in (1), is a scanning lens in which the opposite surfaces are both anamorphic surfaces, and hence there are problems to be solved in that:                beam spot diameter thickening occurs due to decentering; and        when the special toric surface is used for a surface having a large angle of inclination, machining accuracy deteriorates, thereby causing a defective image having thickened beam spot diameter due to a form error such as swells, or vertical lines.        
In the scanning lens using the conventional special toric surface, as described in (1) and (2), the lens is thick, and even if the lens is a plastic molded article, the forming time and the cost of parts increase. Further, shading and ghost light have not been taken into consideration.
Further, in the scanning lens described in (1) and (2), a surface in which the radius of curvature in vertical scanning becomes asymmetric with respect to the horizontal scanning direction is adopted, but a rotationally symmetric aspheric surface is not used.